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The button will allow touch and stylus users to activate Select-to-Speak
without a keyboard, but is also able to be used with a mouse.

This adds a button to the tray that is notified and updates its appearence when
Select-to-Speak state changes. In a following change, the Select-to-Speak
extension will listen to state change requests from the button.

Bug: 753018
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: I3a1ab08c579696cf199c32deec368473952b57fc
Reviewed-on: https://chromium-review.googlesource.com/1040268
Commit-Queue: Katie Dektar <katie@chromium.org>
Reviewed-by: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/master@{#555840}